1. Field of the Invention
This invention relates to a display using an organic electroluminescence (hereinafter referred to as the organic EL), in particular to an organic EL display for color display having an improved light extraction efficiency in a simple fabrication process and a method of fabricating the same.
2. Description of the Related Art
The organic EL display comprises self-emitting organic EL cells placed on a substrate of glass, etc., to emit light thereby to display information. The organic EL display is advantageously thin and light in weight, high in image quality, superior in dynamic image display, wide in the angle of view and low in power consumption as compared with the other types of thin displays. The organic EL display, therefore, is considered a promising candidate for the next-generation thin display supporting the ubiquitous society.
The organic EL display is configured of a plurality of organic layers held between an anode and a cathode. The organic layers, which may be made of a low-molecular material, often include a plurality of function layers such as a hole transporting layer and/or a hole injection layer or an electron transporting layer and/or an electron injection layer on both sides of an emissive layer.
Upon application of a DC voltage between the anode and the cathode sandwiching the organic layers of the organic EL cell, holes and electrons are sent into the emissive layer through the layers described above. The holes and the electrons are recombined with each other in the emissive layer and generate energy, which excites the electrons of the organic molecules contained in the emissive layer. While this very unstable electronic state falls to the ground state, energy is generated as light so that the organic EL cell emits light. This luminescence principle is shared with the light emitting mechanism of the light-emitting diode (LED), and therefore the organic EL cell is also called the organic light emitting diode (OLED).
The method of driving the organic EL cell is roughly divided into the passive matrix method and the active matrix method. In the passive matrix method, band-shaped anodes and cathodes are laid horizontally and longitudinally in array, and the organic EL cell held at each intersection is selectively driven to emit light. In the active matrix method, on the other hand, each pixel is composed of a thin-film transistor (TFT) having the dual function of a switch and a memory whereby the organic EL cell emits light. The active matrix method in which light is positively emitted from the pixels using TFT, though higher in fabrication cost than the passive matrix method, is low in power consumption and high in brightness.
The method of extracting light from the conventional organic EL display, on the other hand, is divided into the bottom emission method and the top emission method. In the bottom emission method, as shown in FIG. 4(a), light is extracted from the insulating substrate 2010 side. In the top emission method, on the other hand, as shown in FIG. 4(b), light is extracted from the upper surface side of the cathode 1014. In the organic EL display 1001 of active matrix type, the TFT is formed on the substrate, and therefore the top emission method can more advantageously improve the light extraction efficiency by increasing the aperture ratio.
The full-color display mechanism of the organic EL display is basically identical with that of the liquid crystal display. Specifically, subpixels of the three primary colors, i.e. red, green and blue (which may hereinafter be referred to as RGB) are arranged on the substrate and various colors are produced by color mixture in accordance with the outputs of the three primary colors. In the color organic EL display, therefore, three types of organic EL cells for emitting blue, green and red light are arranged sequentially as subpixels on the substrate.
To increase the brightness of the organic EL display, the light emitted from each organic EL cell is required to be extracted as much as possible. The organic EL display is a device emitting light in a very thin organic film of about 1500 Å, and the light expands in every direction at solid angle of Ω=4π in the film. In the organic EL display 1001 of top emission type, as schematically shown in FIG. 3(a), light is normally emitted through the cathode 1014, while at the same time the light emitted to the anode 1012 is reflected on the anode 1012 and also emitted from the cathode 1014 side. In the organic. EL display of bottom emission type, on the other hand, the functions of the anode and the cathode are simply reversed. The organic EL display 1001 of top emission type, therefore, generally employs the anode 1012 as a reflective electrode high in reflectivity and the cathode 1014 as a transparent electrode high in transmittance.
Further, in order to improve the light extraction efficiency of the organic EL display, the light leaving the transparent electrode directly and the light leaving from the transparent electrode after being reflected on the reflective electrode are required to reinforce each other. Let L be the optical reflection distance between the light emitting portion having the highest light emission intensity in the emissive layer and the reflective electrode, and φ be the phase change of light reflected on the reflective electrode. The conditions for extracting the light having the wavelength λ at high efficiency utilizing the reinforcing effect of interference is given as L+λφ/4π=(2m+1) λ/4, m=0, 1, 2 . . .
where di is the thickness and ni the refractive index of the ith function layer. Then, L=Σ(i)nidi. Unless the thickness of the function layers included in the organic layers for each cell is set in an appropriate range as shown in FIG. 3(a), therefore, a sufficient brightness cannot be secured. Especially, as long as the function layers for R and B having a large difference in wavelength are prepared with the same thickness, it is difficult to obtain characteristics satisfactory for both colors without sacrificing one of the colors. Since R is larger in wavelength than B, for example, the function layers for R are preferably thicker than those for B to increase the optical reflection distance L.
Thus, the color organic EL display having low-molecular organic layers is fabricated in the following manner, for example, taking advantage of what is called the mask process. In the following description, FIG. 3(a) or FIG. 7 is referred to and the metal mask is used.    (1) An insulated substrate 1010 is prepared, and patterned with a reflective electrode 1012 by sputtering using the metal mask. The reflective electrode 1012 is formed of an Al mixture or Cr, etc. and makes up an anode, for example.    (2) Next, the metal mask is set in position on the reflective electrode 1012 of the red subpixel to deposit organic layers on the reflective electrode 1012. Normally, the metal mask opens only above the red subpixel, for example, and can be deposited by evaporation with only the subpixel of the particular color. In FIG. 3(a), a hole injection layer 1020, a hole transporting layer 1022 and a red emitting layer 1018 of a predetermined thickness corresponding to red light are stacked by vacuum deposition, for example, through the metal mask. Then, the metal mask is fixed, and an electron transporting layer 1024 and an electron injection layer 1026 of a predetermined thickness corresponding to the wavelength of red light are stacked on the red emitting layer 1018 by vacuum deposition.    (3) Next, the metal mask is slightly moved, and set in position in such a manner that the organic layers can be deposited by evaporation on the reflective electrode 1012 making up the green emitting layer, for example. The function layers including the hole injection layer 1020, the hole transporting layer 1022, the emissive layer 1018, the electron transporting layer 1024 and the electron injection layer 1026 of a predetermined thickness corresponding to the green light wavelength are stacked in this order by vacuum deposition.    (4) Next, the metal mask is set in position so that the organic layers can be deposited by evaporation on the reflective electrode 1012 forming a blue emitting layer. Subsequently, like in the process described above, the function layers of a predetermined thickness corresponding to the blue light wavelength, with the blue emitting layer 1018 held therebetween, are vapor-deposited sequentially through the blue emitting layer 1018.    (5) Finally, the metal mask corresponding to each subpixel is removed, and the transparent electrode 126 of ITO (indium tin oxide), etc. is deposited over the entire surface. In this case, the transparent electrode functions as a cathode.
The use of the complicated mask process described above makes it possible to fabricate the organic EL display in which the emitted light can be extracted with high efficiency using the mutual reinforcing effect of interference. As long as a low-molecular material is used as a light emitting material and formed separately for each color with a metal mask, however, all the organic layers are required to be formed separately from each other using the masks for each color. Especially in the case where the organic EL display of top emission structure is fabricated advantageously to increase the light emitting area, all the organic layers are required to be separately formed from each using a mask for each color to achieve both a high color purity and a high emission efficiency at the same time in view of the fact that the range of an appropriate thickness of each organic layer is varied depending on the light extraction efficiency for each of the colors of R, G and B. As a result, the time required for the vapor deposition of the mask is increased, which not only reduces the productivity but also increases the fabrication cost.
In the case where all the organic layers are formed separately using the metal mask, various problems are posed by the forming time, the material utilization factor, the mask cleaning frequency, etc.
In the top emission structure, however, the anode is often used as a reflective electrode, and the hole injection layer and the hole transporting layer can be formed of the same material for RGB between the anode and the emissive layer. In the fabrication process, therefore, these layers are formed far more advantageously collectively without using the mask.
Japanese Unexamined Patent Publication No. 2000-323277 discloses a multicolor organic EL display in which predetermined function layers included in the organic layers formed between the emissive layer and the reflective electrode are collectively formed in the same process.
In the organic EL display described in Japanese Patent Publication No. 2000-323277, however, the function layers included in the organic layers interposed between the emissive layer and the transparent electrode are formed in the same process, while the remaining organic layers (supplementary layers) interposed between the emissive layer and the transparent electrode have different thickness for different subpixels. In Patent Publication No. 2000-323277, therefore, the organic layers are simply set to an appropriate value for each subpixel and the mask process according to the prior art is not effectively reduced.
Accordingly, it is an object of this invention to provide an organic EL display and a fabrication method thereof high in light extraction efficiency, in which the organic layers interposed between the emissive layer and the reflective electrode can be formed collectively for a plurality of subpixels, and the fabrication cost is reduced with an improved productivity by reducing the number of mask processes.